Damage to the articular surfaces or to the ligaments of the knee changes the patterns of movement of the bones on each other and the response of the joint to load. Osteoarthritis follows from failure of the cartilage in one or other of the three joints, leading to bone-on-bone contact and the onset of pain. Frequently, osteoarthritis first manifests itself in the medial compartment, while the ligaments remain intact. The disease can remain confined to the medial compartment until the anterior cruciate ligament fails and the disease then spreads to the other two compartments. No drug treatment has been found which reverses these processes.
Total knee replacement is the most common surgical treatment for osteoarthritis, involving replacement of the articular surfaces of all three compartments and sacrifice of some of the ligaments. Partial knee replacement, for example a unicompartmental knee artheroplasty, involves replacement of the articular surfaces in only one compartment, leaving intact the surfaces of the other two compartments and all of the ligaments. Partial knee replacement can act prophylactically, reducing the rate of development of the disease in the other compartments. Partial knee replacement is surgically more demanding and is not always used when it is indicated.
Mobile bearing arthroplasty may involve fixing metal components to the tibia and the femur. A plastic bearing, an analogue of the natural meniscus, may be positioned between, but not attached to, the metal components fixed to the bones. The metal components are fixed to the bones so as to leave a constant gap between them when the knee is flexed and extended. The surgeon then selects the most appropriate thickness of bearing to fill the gap. The bearing is stuffed between the metal components against the resistance of stretching ligaments. The bearing snaps into position once a thick section of the bearing has passed through the thinnest section of the gap between the fixed components.
A complication that may be associated with mobile bearing arthroplasty is dislocation of the bearing. Dislocation rarely occurs after medial partial knee replacement but is the main complication of lateral arthroplasty. Complete dislocation can occur along the antero-posterior axis of the knee replacement. This can happen either in the anterior direction, as the reverse of the process of implantation, or in the posterior direction. If the patient distracts the joint, i.e. by applying appropriate varus or valgus load to the limb, the bearing may be free to move through an enlarged minimum gap between the femoral and tibial components, which may then come into contact.
Dislocation of a mobile bearing from the lateral side, i.e. towards the inside of the knee, occurs rarely, but could cause damage to the soft tissue. Consequently, it is desirable to mitigate the risk of dislocation of the mobile bearing arthroplasty from the lateral side of the knee prosthesis.
In some circumstances, following dislocation, for example as a result of further degeneration of the knee joint, it becomes necessary to replace the mobile bearing arthroplasty. At revision of these cases, the metal tibial and/or femoral components are usually found to be firmly fixed to the bones, so it is common for the surgeon to replace the worn or damaged mobile bearing arthroplasty with a new bearing of increased thickness, thereby retensioning the joint with a thicker bearing to account for any wear of the prosthesis and/or stretching of the ligaments that may have occurred. However, in cases of severe degeneration it may be necessary to replace the mobile bearing arthroplasty with a fixed bearing arthroplasty. Such a procedure may involve the replacement of the metal component fixed to the tibia, which is an involved and invasive procedure.
Success of the operation, especially that of mobile bearing arthroplasty, depends critically on the presence of intact ligaments. However, the condition of the ligaments cannot be determined with absolute certainty prior to surgery and the decision to use a mobile or fixed bearing can only be made when the joint has been exposed surgically. A typical surgeon is usually an exponent of either fixed or mobile bearing arthroplasty so that, if on exposure, the joint is found to be unsuitable for mobile bearing arthroplasty, the only alternative immedialely available to the surgeon is to perform a much more invasive total knee replacement (TKR). It is desirable to be able to use the same bone preparation techniques to implant both a fixed and mobile bearing prostheses. Equally, when the bones have been cut to accommodate a mobile bearing prosthesis, and the tibial and femoral components have been fitted, it can prove difficult to stabilize the joint with a mobile bearing. It would be useful, therefore, to be able to fix the bearing to the tibial component at that stage.
The present invention seeks to address these issues.